The aerodynamic coeﬃcients Cx , Cy , Cz , Cl , Cm , Cn are primarily a function of Mach number M , Reynolds number , angle of attack α and side-slip angle β and are secondary functions of the time rate of change of angle of attack and side-slip angle and the angular velocity of the airplane. These coeﬃcients are also dependent on control surface deﬂections; otherwise, the airplane would not be controllable. , landing gear, external tanks,) and ground proximity eﬀects. Because of the complicated functional dependence of the aerodynamic coeﬃcient, each coeﬃcient is modeled as a sum of components that are, individually, functions of fewer variables.

The resultant aerodynamic force produced by the motion of the aircraft through atmosphere is resolved into components along the wind axes. The component along the x-axis is called the drag D. It is in the opposition to the velocity and resists the motion of the aircraft. The component along the z-axis (perpendicular to the aircraft velocity) is called the lift L. The lift is normal to an upward direction and its function is to counteract the weight of the aircraft. It is the lift that keeps the airplane in the air.